You may have several issues occurring.

First, are you sure these are the same swans from last year? It is possible that they may be two male swans (yes the males build the nest), but obviously, no eggs. Or, they could be juveniles and are not old enough to mate and produce eggs.

Second, predators (human or animal) could be stealing or destroying the eggs.

Third, there is a possibility that the swans are getting into the twilight of their life and may no longer be producing eggs.

Fourth, the weather may be a factor this year. Several of our consulting sites are showing exactly what you are describing. Eggs are being laid, abandoned, eggs not being produced or nesting has been either delayed or began early and now nothing is happening.

Fifth, if there is not a suitable nesting habitat, i.e., food resources, etc., or there is not enough resources for the entire flock, swans as well as other animals/birds will know when it is not conducive to produce young. So, if something has happened to the habitat this year, i.e., too much snow, late snow, too much rain, drought, etc., that may have disrupted food or other resources, the swans may not produce young.

As the weather continues to get warmer and the hormones settle, the chances that nesting will continue is going to become slimmer. The Regal Swan